“Our benchmark of a 60 percent graduation rate in June is nonnegotiable,” ­Patricia Cuti, assistant principal for guidance, insisted in a June 1 missive to staff.

With fewer than two weeks before the end of regular classes, all 150 students were enrolled in Apex Learning online courses, where they could get a quick and easy replacement for a semester or year’s work.

“With 150 students failing a grad requirement, we are looking at a rate of less than 50 percent,” Cuti wrote. “Every effort to assist these students in reaching all the graduation requirements is necessary.

“I urge you to call the homes as well. ENCOURAGE these students. The ONLY message they need to hear is that THEY CAN and THEY MUST!” she wrote. “Summer graduation is NOT an option.”

Cuti also ordered staffers to come up with ways to fix failing marks.

“Teachers are to speak with these students and discuss a plan to pass the class,” Cuti wrote. “This is extremely urgent.”

Flushing HS is one of 94 low-performing “Renewal” schools that Mayor de Blasio has vowed to fix.

A veteran retired teacher blasted the 11th-hour ultimatum as “too little, too late” to allow for real learning — and called Cuti’s e-mail a veiled threat: “Do it or else.”

But it did the trick, one Flushing teacher said.

“After two to three weeks of working with the computers, students accumulated all needed credits in various subjects, bringing the graduation rate to almost 55 percent,” the teacher said — adding that Flushing HS “feels pressure” from its ­superintendent, Aimee Horowitz, to show progress.

Critics said Flushing’s actions smack of the “Easy Pass” scam recently exposed at John Dewey HS in Brooklyn. After a yearlong probe, the Department of Education confirmed teacher complaints that administrators gave full credits to failing students who completed a “packet” of work with little or no instruction.

Apex courses — also given this summer at Flushing and other schools — let kids rack up credits quickly.

At Bryant HS in Astoria, a teen who failed his English class last semester told The Post he expects to whiz through the Web course in eight or nine days.

“It’s just trying to help students get easy credits,” he said, explaining it was easy to cheat. “You’re not supposed to look up ­answers on the Internet, but people do it anyway.”

City Department of Education officials said preliminary data do not show a “significant increase” in Flushing’s June graduation rate, but gave no figure.